Besides applications, wherein classical activated carbons are used as a mass product, applications requiring special high-performance activated carbons are becoming increasingly important. These are applications, wherein the required amounts of activated carbon for a certain purpose and a certain time of use have to be kept low, and nevertheless excellent adsorption properties (adsorption kinetics, capacity) are required. These are in particular mobile applications, such as for filters in vehicles (cars, aircrafts, etc.) or in gas masks, but also in building air filters. Besides a favorable capacity/weight ratio, other requirements also play a role, such as a low pressure loss over a filter containing the activated carbon. This also means, however, that it is not always possible for in so far additional requirements to use activated carbons maximized with regard to the BET surface; rather, it may be necessary to use activated carbons, which in spite of a moderate BET surface have nevertheless outstanding adsorption properties. In any case, excellent adsorption properties are required, in particular in the case of filters, which are intended to protect persons from toxic gases. In addition, it is desired that a spherical activated carbon is particularly abrasion resistant.
From the document EP 0 326 271, a method for producing an activated carbon is known in the art, which can be prepared from a polysulfonated copolymer. The obtained activated carbon has a multimodal pore size distribution, i.e. a high share of mesopores and macropores.
From the document WO 96/21616, a method for producing an activated carbon from monosulfonated copolymers is known in the art.
From the document WO 99/28234, a method for producing an activated carbon from styrene-divinylbenzene copolymers is known in the art, and by variation of parameters of the method, the pore size distribution can be adjusted in a wide range.
From the document U.S. Pat. No. 4,957,897, a method for producing an activated carbon from a polysulfonated vinylaromatic copolymer is known in the art, wherein the carbonization is effected under dissociation of sulfonic acid groups.
From the document WO 2004/046033, a spherical activated carbon is known in the art, which has an improved pore size distribution and a relatively high fractal dimension.
The educt often used in the above methods is a fresh or spent ion exchanger of a spherical shape. This is a (co)polymer, which carries chemically active groups, for instance sulfonic groups, and has a porous structure or a gel structure. The man skilled in the art knows such ion exchangers from the practice.
All above prior art methods for the production of activated carbons have the common drawback that the absorption properties do not yet meet the highest requirements. This has to do with the following.
Due to the complexity of a rough, in particular microrough, surface, statements about the area of the surface are problematic. The area namely depends on the resolution employed for the determination of the area. The topological dimension of an area is always 2 (topological dimensions are always whole numbers). In contrast thereto, the Hausdorff-Besicovitch dimension or fractal dimension may have a value >2, because of the Szpilrajn theorem, provided that the area has a structure, in particular a microstructure. For an area, the fractal dimension is however always less than 3, since the spatial dimensions are quantized, and thus arbitrarily small self-similar structures cannot exist. In the practice of the gas adsorption, the upper limit is given by the dimensions of adsorbing sample molecules. The closer the fractal dimension comes to 3, the finer is the structure and thus the more “microrough” is the surface. In the case of carbon surfaces, such a microroughness leads to that to a higher degree bondable or at least attractively acting irregularities of the electronic density-of-states functions occur at the (inner) surface with the consequence of an improved bond of molecule species to be adsorbed. The improvement of the bond comprises on the one hand an increase of the packing density within an adsorbed monolayer and on the other hand a higher bond stability. Therefrom results that the setting of a fractal dimension as high as possible in connection with a high microporosity will lead to improved adsorption capacities. This is not contradictory, since a small mesoporosity and macroporosity theoretically reduces the fractal dimension, however the overall contribution of the mesoporosity and macroporosity to the fractal dimension is relatively small.
As a result, in particular the fractal dimension for prior art activated carbons is still improvable.